Ionospheric doppler measurements made at the carrier frequency of an oblique radio transmission provide a very sensitive means of measuring ionospheric phase path variations, but preclude the determination of phase path, from which a reflection height could be inferred because of phase ambiguity. Phase path P defined with reference to the transit time T.sub.p, between a transmitter T and receiver R, of a surface of constant phase, may be expressed as ##EQU1## WHERE .mu. IS THE REFRACTIVE INDEX, .beta. THE ANGLE BETWEEN WAVE NORMAL AND RAY DIRECTION, AND DS IS AN ELEMENT OF THE RAY PATH ALONG WHICH THE INTEGRATION IS PERFORMED. In solving equation (1) it is necessary that the relevant properties of the medium are known such that radio-rays, originating from the transmitter, T, and reaching the receiver R via the medium, can be specified. In an experimental setup the reverse procedure is prevalent, when it is of interest to deduce a reflection height from propagation data without knowing the detailed specifications of the propagation medium.
A method has been developed that permits determination of ionospheric reflection height from doppler measurements. For this method it is required that doppler signatures .delta.f(t) of at least two transmissions, involving for example, one single-hop and one multi-hop, are simultaneously detectable. The novel feature is that phase-path variations, caused by a moving reflection boundary, can be utilized to determine the reflection height by means of the rate-of-change of phase-path information of two transmissions involving a pair of available transmission paths being comprised of multi-hops or a single- and a multi-hop.